1. Field of the Invention
The present invention relates to a total luminous flux measurement apparatus and a total luminous flux measurement method both for measuring a total luminous flux emitted from an object, in particular, to a configuration suitable for measurement of a total luminous flux emitted from a bar-shaped illuminant.
2. Description of the Background Art
Conventionally, a total luminous flux (lm: lumen) has been used as an index for evaluating performances of light sources used in lighting apparatuses. A spherical integrating photometer employing an integrating sphere is known as an apparatus for measuring a total luminous flux with higher accuracy. In the integrating sphere of the spherical integrating photometer, a light source is placed and lit up. A luminous flux from the light source therein is repeatedly reflected by a diffusing and reflecting material (such as barium sulfate or PTFE (polytetrafluoroethylene)) applied to the inner wall of the integrating sphere. This repeated reflection provides a uniform illuminance to the surface of the inner wall of the integrating sphere. The illuminance of the surface of the inner wall of the integrating sphere is proportional to the total luminous flux from the light source. Based on this fact, the illuminance of the surface of the inner wall of the integrating sphere is measured, and the measured value is compared with an illuminance obtained in advance by using a standard light source. In this way, the total luminous flux from the light source subjected to the measurement is determined.
As a prior art concerned with such a spherical integrating photometer, Japanese Patent Laying-Open No. 07-146175 discloses a total luminous flux measurement apparatus including an integrating sphere and a cooling device for the integrating sphere. An object of this total luminous flux measurement apparatus is to achieve stable and accurate measurement by cooling the integrating sphere to a certain temperature when a lamp to be measured is lit up with a constant electric power.
Japanese Patent Laying-Open No. 06-167388 discloses an integrating photometer that employs a hemispherical type integrating unit including an integrating hemisphere having a hemispherical inner wall to which a light diffusing and reflecting material is applied, and a planar mirror extending through the center of curvature of an inner hemisphere of the integrating hemisphere and provided to cover an opening of the integrating hemisphere.
By using each of the above-described spherical integrating photometers, a total luminous flux from a bar-shaped light source such as a straight tube fluorescent lamp may be measured. In this case, however, the integrating sphere needs to have an inner diameter 1.2 times greater than the length of the light source. A reason for this is as follows. As a light emitting portion of the light source comes closer to the surface of the wall of the integrating sphere, light reflected by the surface of the wall comes back to and enters the light source to cast a shadow, which introduces an error to a measured value.
For example, in order to measure a total luminous flux of a 40 W (watt) straight tube fluorescent lamp generally used in schools and offices and having a longitudinal length of approximately 1.2 m, an integrating sphere having its inner diameter of 1.5 m or greater is required. Meanwhile, a 110 W straight tube fluorescent lamp generally used in station buildings and the like has a longitudinal length of approximately 2.4 m. Hence, in order to measure a total luminous flux of such a straight tube fluorescent lamp, a large integrating sphere having an inner diameter of 3 m or greater is required.
As such, a relatively large integrating sphere is required for evaluation of a total luminous flux of a bar-shaped light source such as a fluorescent lamp. This requires effort and cost in terms of its fabrication and transportation. In addition, sometimes it is difficult to secure a space sufficient for installation of such a large integrating sphere.